Cab signal apparatus



Feb. 23, 1954 c. B. SHIELDS CAB szcmu, APPARATUS Filed June 18, 1949 s B. Shields Q INVENTOR.

HIS Ammomwz' I l l I l I l l l I I I l I l I I l I L- T E kum um wawmfimkga ag Patented Feb. 23, 1954 rarest OFFICE.

Y SIGNAL Armin-Arcs QhitlflBS-Bg Shields, Penn. Township, Allegheny County, Pa, assignor to. Westinghouse. Air. Bmkcflompalhsi, a. corporation of Pennsylvania I ApplicationJimalB, 194$, SerialNo. 106;0 27

moiet es.

- .My;inventio1i.relates to-cab signal; apparatus, andmore particularly; to; appa atus. r operatin both. cab signal. apparatus-responsive to coded; en.- ergy. and cab signal-apparatus responsivetononcoded energy.

There are two generalptypesof'; continuous inductive cab signaLsys-tems inextensive-use, One system gusesalternating current coded-by; being periodically interrupted or; varied atone or more code, rates and the other system ucesnon-coded adternatingcurrent. In both of these types of systems trackz circuitsare used, thetrac circuits being; provided with means for supplying-to the rails at the exit end of-the track section an alternatin .current which; is, governed according-to 'stal1ed. On such arailroad a locomotive. which is to be operated over. the-.difierentdivisions must beequipped: with: both f orms,;.of-1 receivingv apparatus if the cab. signals are; torbe used-i; ,Again-,.-a railroad may have a. stretch t. trackway. that --is used by thedifierently; equipped locomotives; A1 so, a railroad having a. cab. si nalinstallation in which non-coded altcrnating current used may find-it desirable to convert. suchaninstallation tea system using. coded: alternating current or vice versa. Thcccnversion of such an.installation requires some period ofi-time and-during; the conversion period the locomotives -must be i. ipp d ith b t the dandfienew fo m of receiving apparatus ii cab signal protectionis to be obtained during the conversionperiod.

In view. of the foregoing conditionsa main object of my invention is theprovision of improved apparatus which is operable to. efiectively influence both train carried receiving apparatus responsive to coded alternating current. and receiving apparatus resp hating current.

A more specific feature of my invention is the provision of improved trackway apparatus operable to cyclically supply to'the; rails of'atrack section alternating currenthaving'a predeteronsive to .nonl-co'cled altermined root mean. square value. and'alternating current having airoot mean, square value which is-..higher than saidnredetermined valueby a given variation. In other words, during one half ofzacode-period alternating current of a given low energylevelis. supplied to the rails and during the-second-h'alf of the code, period alternating current of a. given high; energy level; issupplgied to the. rails, the diiierencehetwecn the low and high, energy levels being at Ieastecual. to a, given predetermined variation.

. Otherfeatures, obj-ectsandadvantagcs or my invention .will appearas; the. speoiiicatior v prorosses.v

In practicing: the inventionthe. locomotives which operatev over a division of; a. railroad equipped witha ccdedjcah si nal: system would be equipped with receiving apparatus responsive tosuchccded alternatin current and; the. loco.- motives which operate. over adivision on which. a cab. .signa1systemusinsnon-coded energy is installed would beeq ipu d with receivin apparatusresponsive to. non-coded; energy. A territory. or stretch, ofctraok which is to be used in common. by; the diiierent locomotives would be quipped with tra hway apparatus embodyin the-invention and which trachway ap ratus is operable,- to supply. alternatingacurrent in a form that efiicctively fluences both tyncsof train carried'receiving apparatus.- he ack of this territory or, stretch. that is to be. used by all of the locomotives iS;.f0 .'m;B d.iD- Q track sections ach orwhich is. rovidedwith. tra k cir uit a pa atus that. includes ahattery and a transformer as a s ur soh supply, the ransformerhavins thre windings one. of: which is. en r ized from an lternating current. source of a convenient frequency, and he other two of which windings are used s se ondary windin s. One of the secon ary; windin s sn os rtioned to supply a relative y low ol asc th oth econ ary windsisurcpc cd to suph a ela ively hi h voltage. The low voltage secondary Winding is o nnectedacross th i sin se ie W hth track batte y o er-a fi s p tion on o a o i means. The. high voltage secondary winding is connected acrossthe rails over a second position contact of the codin me ns= Thus al w ner y level. alternating current is superimposed on the direct currentof: the battery during the one half of each code period that thefirst position contact of the coding. meansis closed and alternating-current of a high energy. level is supplied to the rails during'the seoondhalfof each code period that thes'e'cond position contact of the coding means is closed. This results in an alternating current flowing in the rails at all times. During one half of each code period the energy level of the alternating current is of a low level and during the other half of each code period the energy level raises to a relatively high value there being at least a predetermined variation between the high and low energy levels of the alternating current. The low energy level for the alternating current is made sufficient to effectively influence the non-coded train carried receiving apparatus while the variation between the high and the low energy levels of the two half code periods is made sufficient to effectively influence the code responsive train carried receiving apparatus.

I shall describe one form of apparatus embodying my invention and shall then point out the novel features thereof in claims.

In the accompanying drawings, Fig. l is a diagrammatic view showing one form of apparatus embodying my invention when applied to a stretch of railway through which locomotives equipped with train carried receiving apparatus responsive to coded alternating current and locomotives equipped with train carried receiving apparatus responsive to non-coded alternating current operate. Fig. 2 is a diagram illustrating the characteristics of the electromotive forces applied to the train carried receiving apparatus by the trackway apparatus of Fig. 1..

Referring to Fig. l, the reference characters la and lb designate the track rails of a railroad over which tranic normally moves in the direction indicated by an arrow and which rails are formed into track sections by the usual insulated rail joints. Only the one section Dli and the adjacent ends of the adjoining sections are shown since these are sufficient for a full understanding of my invention. Each section is pro- 'vided with a track circuit which includes a track relay connected across the rails at the entrance end of the section and a power source mean connected across the rails at the exit end of the section. The track relay is a direct current code following relay identified by the reference character TR plus a prefix corresponding to its location. The power source means or each track circuit includes a track battery and a track transformer together with a coding means. The track battery is identified by the reference character B plus a prefix corresponding to the location and the transformer is indicated at T plus a prefix corresponding to the location. The coding means will be more fully described shortly.

Each track relay governs code detecting means which as here shown comprises two slow release relays and a decoding unit, the relays being identified by the reference characters P and H plus a prefix corresponding to the location and the decoding unit being shown conventionally at DU. Thus at each junction of the track sections there is located a traclaway apparatus which comprises the track relay and its associated relays for the section in advance and the power source means for the track circuit of the section to the rear. In addition, the trackway appa ratus at each junction location includes an appreach controlled relay which is identified by the reference character AR plus a prefix corresponding to the location.

a source, the terminals of which are indicated at B and N, over a circuit completed at the front contact 10 of the track relay DTR. A second slow release relay DH is supplied with an energizing pulse of current over a circuit including back contact 18 of the track relay DTR and front contact H of the relay DP. Consequently the relays DP and DH are recurrently energized in response to code operation of the relay DTR and are retained picked up due to the slow release characteristics of the relays.

The relay DTR. also governs a decoding unit DU over a contact it, the arrangement being such that the decoding unit DU is e'iiectively energized only when the relay DTR is operated at a preselected code rate which, as will appear hereinafter, is taken to be a rate of 18-) cycles per minute.

The relay DH governs the supply of current to the track circuit of section D E. With relay DH picked up, a circuit is formed for supplying coded current to the rails of the section D-E, the circuit being traced from the positive terminal of battery DB through a first secondary winding 13 of track transformer DT, a front or first position contact it of a coder lfitCT, front contact of relay DH, lead wire it, rail ii) of section D-E and thence through the rail ib, assuming that the train shown conventionally at IN! is not present in the section D--E, the winding of track relay ETR at the entrance end of the section, rail la, lead wire ll and a limiting resistor 3 to the negative terminal of battery DB. The coder iSilCT may be any one of several known form and it is here shown as of the relay type having an operating winding which is continuously supplied with current from the BFN source, and which coder when thus energized is operated to actuate the contact member alternately to engage the first position contact is and a second position contact 23 at a selected code rate. Preferably the coder IEBCT is proportioned to operate its contact member at a code rate of 180 times per minute, such code rate being that in common use in cab signal systems, but some other code rate can he used. A primary winding 2d of the transformer HT is connected to a suitable source of alternating current, the terminals of which are indicated at BX and NX through a circuit completed at back contact T3 of relay DAR. This source of alternating current may be of any suitable frequency such as the usual it cycle commercial current,

or it may be of a frequency of the order of cycles per second which is commonly used in railway signal installations. Thus, an alter hating current pulse from the secondary winding it of the transformer U1 is superimposed on the code pulse of direct current from the battery DB during the code period that the contact M of coder IBBCT is closed when the approach controlled relay DAR is released. The direct current component of this pulse of energy supplied to the rails causes operation of the track relay ETR, and the alternating current component is available for control of cab signals as will appear when the operation of the apparatus is described.

With the track relay ETR for the section D-E following code, its associated relays EP and ill-I are energized and picked up in the same manner that the relays DP and DH are picked up in response to code operation of the track relay DTR.

-Furthermore, the track relay ETR governsa decoding unit; DU'i-n the samea manner that the track-relay DTR governs its decoding unit DU.

r the" events the: approach controlledv relay DAR at location-D deenergizedandreleased, a mannerxtobe explained hereinafter, ;a circuit forconnect-ingi the secondary winding 32- of'the track transformer DT to the rails .is'. completed, this circu-it being traced from the right-hand ter, minal of secondary winding 32 through. lead wires 33 and I1 to-therail ta, and-tromthe left-hand terminal of the secondary winding azthrough resistor 34, back contact '3-l-of relay DAR, second position contact 2i: of coderv I BEICT, front contact I5 otrelay- DH and lead wirelfi to rail lb.- Thus, under this condition of the apparatus at. the location-:D, pulses of alternating currentfrom the secondaryw-indingtlare supplied to the rails during the half: code periodthat. the coder contact.23: is-c1osedi The utili 'ty of this code pulse of alternatingcurrentwiil alsohe explained hereinaften. V 1 i Y In the event thesectioninadvance of section DE is occupied and the track. relay DTR is shunted, the relays DP and DH. are released,. and

with relay DH released closing its back. contact ii, a pulse of direct current from the. battery DB is supplied to the rails of the section D-E through. a. connection which extends from the positive terminal of battery DB through a front or-first position contact l2 of a coder 150T, back contact ii of relay DH, lead wire l6, rail lb, track relay ETR, assuming the train TN i is not present, rail I a-and lead wire I! and resistor l8 to battery DB. 1 The coder TECT is similar in com struction to the coder IB JG'I? except it is proportioned to operate its'contact. member to alter.- nately engage; its first and second position contacts at acode rate of 75 timesper minute. Thus, when the section. in. advance. of. section D-E is occupied, code pulses of direct current of the 75' code rate are applied to the rails. of the. section D-E but no codedalternating,current is applied to the rails. It follows. that the direct current codefollowing track relay .E IB, is. operatedat the 180 code rate when the section in advance of sectionD-E is unoccupiednandfis operatedat the '75 code rate when the section in advance is occupied. 7

With the track relay ETR operated at either of the code rates, the associat'edicode detecting relays, EP andEI-Iareenergized and pickedup, but the decoding unit DU associated. with the relayETR is effectively energized only when the relay ETR is operatedat the- 180 code rate The relay DH and the decoding unitDU at the location D and the relay EH and the decoding unit at. the location E would be used; for the control of wayside signals not shown, the wayside signals, and the circuits: being. omitted in order to simplify the drawing since thesecircuits and wayside signals are not'a part of my present invention. V

The relay EHand theapproachcontrolled relay EAR atlocation E govern' circuits for the supply of current to the rails of the section next in'the'rearof section D-E in the same manner that the relay DH and the approach controlled relay DAR control the code pulses of current applied to the rails ofthe section 13-33 and the description need not be repeated.

It is to be pointed out that the approach con trolled relay-DAR at location D may be governed by any one of several well-known arrangements of approach control' -and in'th'e present application the relay DAR is normallyenergizedand picked up over a line'- circuit including front contact l2: of? relay EH; soithat' when..,ar train entersut'he; section. D-E. and. relay. EH? releasedthe. relay DAR is .deenergized and released closing its. back contacts 3-]. and 13.. Ina similar manner the approach controlled relay. EAR is normally energizedand. picked up, and is. deenergized andreleasedin. response to a train entering: the section next in the rear of section relationship. to. the railsv and .thus these. inductors pick up. electromotive. fcrceszin response. to: alterhating current flowing inthe rails. Inductors '35and,36 are connected in series in such a manher that the. electromotive. forces induced therein due to alternating current flowing in the. rails in opposite directions. at any given instant. are addie tive. The inductors 35 and 36 are connected acrossthe..inputterminals 31 and 38 of the receiving apparatus. which is indicated .as a, whole by thev dot anddashrectangleTRL. This. receiving apparatus or, receiver TR! for train TN! may be. any one ofseveral arrangements. of receiving apparatus responsive to alternating current periodicallyinterruptedv or varied at a given code rate. .The. receiver TRI. may be, for example, similarto that. disclosed in Letters. Patentof the United States No. 2,462,454,.granted February 22,. 19.49, to LeslieR. Allison, for Train Carried Cab Signal Apparatusand referencev is made to this patent for a full understanding, of such receiving apparatus. Itis sufiicient for the present applicationto point out that the receiving apparatus TRL includes an amplifier unit provided with a code .iollowin'grelay. which is operated in response. to. the. electromotive. forces picked up by theinductors 35 and .35... The relay in turnis used. to govern decoding means whichin turn control cab. signals. and. train control device.

The. amplifying unit .of the. receiving apparatus v'I'lEtLis indicated'asa Whole by the dash rectangle AMI. and this amplifying unit includes as essentialielements an input. or filter transformer FT, an electron tube VT, a master or output transformer MT, and a code. following relayli/LR, together with a proper-source. of; power. Preferable the. power source for thereceiving apparatus 'IRlr is the 32 volt. train lighting source, the terminals'ofwhich are indicated in the drawing at B32 and N32,. terminal B32 being the positive terminal...

The'tubeJVT is. shown asaan indirectly heated tetrode', the heater id of which: is. connected dis rectlty across the power. source so that: the tube isconstantly heated. The tube VT is designed and constructed. for operation from a 32 volt source,- but'it is to -be. understood that'thc invention'is not limitedto this type of tube and other typeset tubes can be used.

'Theinput' transformer FT 'isprovided with a firstwinding' ii which; in series with a capacitor a2, is connected across the input terminals-3'! and 38 for receiving the electromotiveforces from the inductors. A second winding d3 of this transformer is iu'cludedin a tuned circuit comprising control grid 44 of the tuhevTand'a capa'citordfi, the tuhe' VT'being' preferably of 'aconstruction such that two terminals are provided for the control grid 44, one connected to each end of the grid. The two circuits associated with the transformer FT are tuned to resonance at the frequency of the alternating current supplied to the rails and current of some other frequency flowing in the rails is substantially suppressed insofar as the effect on the receiving apparatus is concerned.

A screen grid 48 of the tube is connected to the positive terminal B32 and the tube is provided with an anode-cathode circuit that extends from terminal B32 through a primary winding it of the master transformer MT, anode 48 and tube space to cathode 49, a resistor Bi, and a biasing unit BU to the negative terminal N32. The biasing unit BU consists of a resistor R2 and a capacitor C! in multiple. The winding M is provided with a by-pass capacitor H.

The lower terminal of the winding 43 of the filter transformer FT is connected to the negative terminal of the power source. ihe parts are preferrably proportioned for the anode-cathode current to have a substantially zero value when no electrornotive force is impressed upon the receiving terminals of the apparatus.

The relay MR. is preferably a stick polar relay operated to reversely actuate its polar armature E8 in response to electromotive forces of opposite polarity and of a given value applied to the relay winding, and which winding is connected across a secondary winding 5! of the output transformer MT. Thus the relay MB is operated to one position by the electromotive force induced in secondary winding 54 due to an increase in the anode current flowing in primary winding ii, and is operated to its second position due to the electromotive force induced in secondary winding i3! due to a decrease in the anode current, the relay being operated only when the variation of the anode current equals a predetermined value. In

other words, the relay MB is operated in response to variations in the anode current of the tube VT of a predetermined magnitude. That is to say, the relay MB is operated in response to variations in the direct current component of the anode cur- 1 rent for the tube VT, the alternating component of the anode current being by-passed by the capacitor 'H. Consequently when the anode current for the tube VT is not varied and when the variation is less than a predetermined value, the

relay MR remains at the position at which it was last moved.

As explained in the aforementioned Allison patent the relay MR by its operation governs the decoding means for the cab signal apparatus, the

decoding means being controlled according to the rate at which the relay MR is operated.

Similarly, the train TNZ is provided with two inductors 2! and 22 which are connected to the input terminals 53 and 56 of the train carried receiving apparatus or receiver TR2 for supplying to the receiving apparatus the electromotive forces picked up due to the alternating current flowing in the rails.

The receiving apparatus or receiver for the train TN! is indicated as a whole by the dot and dash rectangle TRZ, and it may take any one of several d fierent arrangements, and it may be similar to that disclosed in Letters Patent of the United States No. 1,568,331, granted December 22, 1925, to Lloyd V. Lewis, for Railway Traffic Controlling Apparatus, refelence being made to this patent for a full understanding of such train carried receiving apparatus.

It is sufiicient for the present application to point out that the receiver TRZ includes anamplifier AMZ and a relay 1-12, the relay H2 in turn governing the cab signals and other train control devices. The amplifier AM2 is shown conventionally and it is of the electron tube type and as disclosed in the aforementioned Lewis atent, it comprises two separate two-stage amplifiers the input circuits of which are connected in multiple across the input terminals 53 and 54 and which amplifiers are arranged for the output of one amplifier to be shifted in phase with respect to the output of the other amplifier.

The relay H2 is of the induction motor type having two stationary windings 55 and 56 and a rotor 5?, and to which rotor a contact member 53 is operatively connected. One winding 55 in multiple with a capacitor 59 is interposed between a power source terminal indicated at B300, and the anodes of the tubes of one of the two separate amplifiers for energization of the winding 55 in response to the alternating current produced in the output of this amplifier. The other 5% in multiple with a capacitor 6!! is interposed between the Biiilii power terminal and the anodes of the tubes of the other amplifier and is energized by the alternating component of the anode current of this amplifier. The connections of the two separate amplifiers are arranged so that the currents supplied to the two relay windings 55 and 5%) are shifted in phase and it follows that the rotor 51 is rotated from its biased position to its other position when the output currents of the amplifiers are of a predetermined value. Consequently, the contact member 58 of the relay H2 is operated from a biased position to a second position when the alternating current supplied to the input terurinals 523 and 5 of the receiving apparatus T32 has a predetermined root mean square value, the contact member 53 being held in the second position as long as the alternating current applied to the input terminals does not fall below this root mean square value.

In describing the operation of the apparatus. I shall first consider the operation of the receiving apparatus for the train TNI, and I shall assome that the train Til is moving through the section D-E when the section next in advance is unoccupied. Under this trafiic condition, the relay DH is picked up and the approach controlled relay DAR is released. During one code period of coder IBGCT, the alternating voltage from secondary Winding I3 is supplied to the rails causing a given value of alternating current to flow in the rails, the current flowing down one rail and through the wheels and axles of the train and back the opposite rail. As explained hereinbefore, the alternating voltage of the secondary winding i3 is of a low value and the electromotive force picked up by the inductors 35 and 35 due to the low value of alternating current when applied to the amplifier AMI creates a variation in the anode current of the tube VT which induces a voltage in the secondary winding iii of the transformer MT of a value too small to operate the relay MR. That is to say, the alternating current supplied to the rails from the secondary winding 3 during this code period is ineffective to operate the relay MR of the train TNI. During the code period the contact 23 of coder 13501 is closed, alternating current from the secondary winding 32 is supplied to the rails. The value of this current is greater than that supplied by the secondary winding 13 and the variation of the anode current caused by the electromotive force picked up by the-inductors is great shifts itscontacts and current is no longer supplied by the secondary winding- 32, and the-de- "creasein the anodecurrent causedto flow in -the tube 'VT is great enough to induce animpulse-in secondary Winding I sufficient *to operate 4 the relay MR back to its first position. -:Consequerr tly the relay MR is operated at the' l 80-code"rate and =a'olear cab signal' is providedto indicate that the section next advance is unoccupied. r

1 shall" next assume that the-section'next in advance of section D--E is occupied-andthe 'relay DH is'released during the timethe trailfi ;T-N 'l is moving through thesection Under this traiiic condition the secondary w-ind-ings la and -32 of the trans/former 7D; ---aredisconnected at from; contact 45 oi-relay DH- andon1y-=pu1sesof "direct current {from the lcsditery *DBj'0f '11he- 75 code are applied to the rails with the --result the relay MRof the train *TNI is inactive and a slow speed cab signal-is provided "to indicatethat :the section-meat in advance-is occupied. Similarly, if another train oocupiessection WEI-inadyance of the train "l i the train in advanceshunts-aH current away. from-the train TN-l and-the relay MR' is inadtive-and asloWwspeed-cab signal provided. '-It isto lee-pointed 'out--that-- the--cab signal system here under consideration is 1 of the type thatprovidesenly the-two indicationsya clear cab signal when at least one section in a-d- Nance is unoccupied and a slow speed cab signal when the section-next un-advance-is-occupied, and-also when theseotion being traveled is0ccupiedby-atrain ahead; I r 7 1 shall next describe the operation 1 of *the receiving apparatus for the train' 2 and in this connection I-shaiil-assumethat :the train is not presentand that the traimTNZ moves from the :position shown-in Fig. -1 to enter the section *while the ---section in advance oi D- E- is-=-unoocupied-. Under this traffic condition the tra-okwayapparatus-at donation functi-ons to isupply alternatingcurrent from the sec- :ondarywindingll3 to the rails during -thepode period the contact 4'4 ot-coder 71 8601 -is-closed and-to supply alternatingcurrentfrem the sec- 'ondary winding 32 of tiansformen ET-*to the rails during the code period that the contact 523 of :coder lBl'iCT is closed. In r-esponse to-this rail current; the eleotromotive ioree picked up by -the inductors -21- and 22* -of-the= train BN2 is of :.the form 'indicated by Fig. 2. I hat is,-during one ihalf: code pel'ioclthealternating-current in the rails has a relativelylmyenergy =1eVe1--and during the other half code period -thetalterna-t- -ing :current iiowing- -in the rails-has a relatively :high .energy *level. The parts ofw-thereceiving apparatus TRiof train :are I proportioned :the output 0f the "amplifiers: when the low energyezlevel' .electromotive :force is applied itodthe input terminalse'fifi and 54: and consequently: it iwillibeiheld iIlzJihiSzlflOSitiDlI '9;1S()iduringthe h1f nndenperiodzdhat; ithe i alternating i-currenti is' o'l the highenergy level. I Thus relay "H2 of r l he train TNi is held in itsoperated-position .,con-

tin-uously during Jthe time the train is .moving --through the section De-xE when alternating cur.-

rent first fromsecondary winding, l3 and-then afromsecondary winding supplied toithe rails through i the A .trackway via-pparatus at localn-theevent the section in advance of section is:-0ccupied and relay-DH is -releasedzwhen the train TNZ moves into :thesection Em-iEUonly code pulses of direct currentaresupplied .to the rails and thus the relay-H2 isldeenergizedand moves :to its .-Ibiased position to -effect-a slow speed-cab signed for the train TNZ.

it is -to be pointed out that :when either train 'TN I :or train TNZ moves .out :ofutheisection and coded direct current-is supplied to the rails, this coded direct current of eithenthe .75 code or-the :18!) code isefiecztiue. to operate theirack relay .ET R,v so that the apparatus is restored-@ its normal position.

Although I have herein shown and described bu -tone form .of .cab signal apparatus embody.- ingvmyl invention, itis understood that 'ivarious changes and modifications maybe made therein avith'in; the-scope of; the appended claims without departing from the spirit and scope of my yinven --tion. Y

I-Iaving .thus claim is: v 1 combination, .a vsection of railway :track through which strains equipped :with icab signal apparatus responsive :toalternating currentfi-ow ingdn. the track rails are operatedyisaidhcab signal apparatus of at -'1eastone ofsaid trains including an inductonan amplifier and a-relay operable in response to a preselected root mean square value of alternatin current of a selected f'requencymfiowing in therails; said cab signal apparatus of atleast one otherof said trains including---an inductor, an amplifier and a relay operable i-n response to :a given code rate of 'oc- -c'urence of a given variation of the root mean square value of the alternating current "of said selected frequency flowing in the-rails; said given --variati0ns being in the -=i'orm of a squareivave type nodu-lation :of said alternating current of said-selected irequencypan alternating'current source of said selected -frequenoy,'a coding device operable to cyclical-1y operate a pair of contacts-at sai'd -given code -rate ,-cirouitmeans including said source and-said pairofcontacts for-cyclically supplyingto the Trails of said section-alternating current-of a highand a low energy level, the modulation of said alternating current being of the square Wave --ty pe said low] energy level of current being'of at least-said pr'eselecte'd root mean square value, and said high energy level of current Joeing root mean square-value greater- -*than said preselected value -by 1 at least saidgivenwariatiom l In combination, a section of railway -track throughwhichtrains equipped-with cab signal apparatus responsive to an alternating electromotive force are operated-,- -eao-h of said trains having -aninductor connected to -i-ts cab s' 'nal apparatus and -mounted for -inductiverela on to-the track rails sa-id cabsignalaapparatus of at least- -one -01" i said .atrains including' an' amplifier and a -relay operable-in response to an alte-r-nat ing electro notive force-of a selected frequency and having at least a given not; mead-square value -=supplied thereto, said cab --sigria1 -apparatuso'f: at least one other-train=incliiding afi amdescribed my invention, whatv l plifier and a relay operable in response to an alternating electromotive force of said selected frequency and having at least a given variation in its root mean square value occurring at a given code rate and in the form of a square wave supplied thereto, a source of alternating current of said selected frequency, a coding device operable to alternately close a first and a second contact at said given code rate, a transformer having a primary winding receiving current from said source and provided with a first and a second secondary winding, circuit means including said first and second contacts to alternately connect said first and second secondary windings to the rails of said section, said first secondary winding supplying current effective to induce in the inductor of a train an electromotive force having said given root mean square value, and said second secondary winding supplying current effective to induce in the inductor of a train an electromotive force having a root mean square value greater than said given value by at least said given variation, said given variation being in the form of a square wave modulation of said alternating electromotive force of said selected frequency.

3. In combination, a section of railway track through which trains equipped with cab signal apparatus responsive to an alternating electromotive force are operated; the apparatus of a certain one of said trains including an amplifier and a relay and being operative to provide a clear signal indication only when an alternating electromotive force of a selected frequency and of at least a preselected root mean square value is applied thereto; the apparatus of a certain other one of said trains including an amplifier and a relay and being operative to provide a clear signal indication only when an alternating electromotive force of said selected frequency and having a given code rate of a given variation in its root mean square value is applied thereto, said given variation occurring in the form of a square wave modulation of said alternating electromotive force of said selected frequency, a source of alternating current of said selected frequency, a coding device operable to alternately close a first and a second contact at said given code rate, a transformer having a primary winding receiving current from said source and provided with a first and a second secondary winding, a first and a second circuit means including said first and second contacts respectively to connect said first and second secondary windings respectively to the rails of said section, each said train pro- P vided with an inductor connected to its cab signal apparatus and mounted for inductive relation to the track rails, said first secondary winding supplying a current effective to induce in the inductor of a train an electromotive force of said preselected root mean square value, and said second secondary winding supplying a current effective to induce in the inductor of a train an electromotive force having a root mean square value greater than said preselected value by at least said given variation.

4. In combination, a track section, a source of alternating current of a selected frequency, a coding device operable to alternately close a first and a second contact at a given code rate, a transformer having a primary winding receiving current from said source and provided with a first and a second secondary winding, 2. first circuit including said first contact to connect said first secondary winding across the rails of said section, a second circuit including said sec ond contact to connect said second secondary winding across the rails of said section, said first secondary winding adapted to supply alternating current of at least a given low energy level to the rails, and said second secondary winding adapted to supply alternating current of at least a given high energy level to the rails and which high energy level is greater than said low energy level by at least said given variation, the form of said variation being on the order of a square Wave modulation of said alternating current.

5. In combination, a track section through which trains normally move in a given direction, a code following direct current relay connected across the rails at the entrance end of said section, a battery, a source of alternating current of a selected frequency, a transformer having a primary winding receiving current from said alteranting current source and provided with a first and a second secondary winding, a coding device operable to alternately close a first and a second contact at a given code rate, a first circuit including said first contact to connect said battery and said first secondary winding in series across the rails at the exit end of said section, a second circuit including said second contact to connect said second secondary winding across the rails at the exit end of said section, said battery supplying direct current for operating said relay when the section is unoccupied, said first and second secondary windings supplying alternating current of a low and high energy level respectively for operating cab signal apparatus responsive to non-coded alternating current of said selected frequency and having at least a given root mean square value and cab signal apparatus responsive to alternating current of said selected frequency and coded to have a given variation in its root mean square value, said given variation being in the form of a square wave modulation of said alternating current of said selected frequency.

6. In combination, a track section through which trains normally move in a given direction, a given one of said trains equipped with cab signal apparatus including an amplifier and a relay responsive to alternating current of a selected frequency and coded by being recurrently varied in its magnitude at a given code rate, said variation to be of at least a given value and in the form of a square wave type modulation of said alternating current of said selected frequency, another one of said trains equipped with cab signal apparatus including an amplifier and a relay responsive to alternating current of said selected frequency and non-coded and of at least a given magnitude, a source of alternating current of said selected frequency, transformer means receiving energy from said source and having a first and a second secondary winding, a coding device having a first and a second contact and operable to alternately close said contacts at said given code rate, a first circuit including said first contact to connect said first secondary winding across the rails of said section, a second circuit including said second contact to connect said second secondary winding across the rails of said section, said first secondary winding proportioned to supply alternating current of at least said given magnitude, said second secondary winding proportioned to supply alternating current having a magnitude which varies from said given magnitude by at least said given value, each of said trains equipped with an inductor connected to its cab signal apparatus and mounted for inductive relation to the rails to supply to the cab signal apparatus an alternating voltage having the characteristics of the alternating current supplied to the rails, said characteristics including said square wave modulation at said given code rate.

7. In combination, a section of railway track through which trains equipped with cab signal apparatus responsive to alternating current flowing in the track rails are operated; said cab signal apparatus of at least one of said trains including an inductor, an amplifier and a relay operable in response to a preselected root mean square value of alternating current of a selected frequency flowing in the rails; said cab signal apparatus of at least one other of said trains including an inductor, an amplifier and a relay operable in response to a given code rate of cocurrence of a given variation of the root mean square value of the alternating current of said selected frequency flowing in the rails, said given variation being in the form of a square wave type modulation of said alternating current of said selected frequency; an alternating current source of said selected frequency, a coding device having a contact controlling member which is cyclically operated to a first and a second position at said given code rate, a transformer,

circuit means including said contact member and said transformer to connect said source across the rails of said section, said circuit means including means controlled by said contact member to cause the alternating current supplied to the rails to be of a low or a high energy level according as said contact member is operated to its first or its second position, said low energy level of said current being of at least said preselected root mean square value, and said high energy level of said current being of a root mean square value greater than said preselected value by at least said given variation. said given variation occurring in the form of said square wave modulation of said alternating current.

CHARLES E. SHIELDS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,566,331 Lewis Dec. 22, 1925 1,679,014 Gilson et al July 31, 1928 1,986,679 Lewis Jan. 1, 1935 2,197,416 Place Apr. 16, 1940 2,219,876 Sorensen Oct. 29, 1940 2,331,815 Thompson Oct. 12, 1943 2,515,642 Gilson July 18, 1950 

